Negative ribbon ion beams from pulsed plasmas

ABSTRACT

An apparatus and method for the creation of negative ion beams is disclosed. The apparatus includes an RF ion source, having an extraction aperture. An antenna disposed proximate a dielectric window is energized by a pulsed RF power supply. While the RF power supply is actuated, a plasma containing primarily positive ions and electrons is created. When the RF power supply is deactivated, the plasma transforms into an ion-ion plasma. Negative ions may be extracted from the RF ion source while the RF power supply is deactivated. These negative ions, in the form of a negative ribbon ion beam, may be directed toward a workpiece at a specific incident angle. Further, both a positive ion beam and a negative ion beam may be extracted from the same ion source by pulsing the bias power supply multiple times each period.

This application is a continuation of U.S. patent application Ser. No.14/811,272 filed Jul. 28, 2015, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD

Embodiments relate to an apparatus and method for extracting negativeion beams from an ion source, and more particularly, extracting anegative ribbon ion beam through the use of a pulsed plasma.

BACKGROUND

Ions are used in a plurality of semiconductor processes, such asimplantation, amorphization, deposition and etching processes. In manyembodiments, positive ions are created from a plasma and are used toperform these processes.

For example, an RF ion source may be used. This RF ion source mayinclude an RF antenna, to which RF power is applied. The RF antenna maybe disposed near a wall of the chamber, which may be made of adielectric material. One or more gas containers may be in communicationwith the chamber so as to supply feed gas to the chamber of the RF ionsource. The excitation of the RF antenna results in the creation ofelectromagnetic energy, which may excite feed gas disposed with thechamber of the RF ion source to create a plasma. Ions from this plasmamay be extracted from the RF ion source using, for example, extractionelectrodes, and directed toward a workpiece. These extracted ions maybecome implanted in the workpiece.

Pulsed electronegative plasmas are promising candidates for improvingetch, deposition processes for precision material modifications andmicroelectronics fabrication. Negative ions can be used to reduce chargebuild-up on the devices and provide “charge free” ion beam processing.In addition, pulsed electronegative plasmas can have extremely highnegative ion to electron density ratio and can be used as a negative ionsource. When negative ions dominate in the afterglow plasma, thenelectron filtering at the extraction aperture is not necessary. Electronfiltering is typically done in a continuous (cw) negative ion sources.

In certain embodiments, it may be beneficial to create a negative ionbeam. Further, it would be advantageous if this negative ion beam can beadjusted, so that parameters, such as neutral to ion flux ratio, anglecontrol, and uniformity, can be readily controlled.

SUMMARY

An apparatus and method for the creation of negative ion beams isdisclosed. The apparatus includes an RF ion source, having an extractionaperture. An antenna disposed proximate a dielectric window is energizedby a pulsed RF power supply. While the RF power supply is actuated, aplasma containing primarily positive ions and electrons is created. Whenthe RF power supply is deactivated, the plasma transforms into anion-ion plasma. Negative ions may be extracted from the RF ion sourcewhile the RF power supply is deactivated. These negative ions, in theform of a negative ribbon ion beam, may be directed toward a workpieceat a specific incident angle. Further, both a positive ion beam and anegative ion beam may be extracted from the same ion source by pulsingthe bias power supply multiple times each period.

According to one embodiment, an apparatus for creating a negative ribbonion beam is disclosed. The apparatus comprises an ion source having aplurality of chamber walls defining an ion source chamber and having anextraction aperture; an RF antenna disposed proximate one of theplurality of chamber walls of the ion source chamber; an RF power supplyin communication with the RF antenna, and outputting a first RF powerlevel for a first time duration to the RF antenna to create a plasmawithin the ion source chamber from a feed gas and outputting a second RFpower level, lower than the first RF power level, for a second timeduration; and a bias power supply to create a voltage difference betweena plasma disposed in the ion source chamber and a workpiece, such thatthe bias power supply is pulsed to create the voltage difference duringat least a portion of the second time duration, so as to extract thenegative ribbon ion beam from the ion source chamber through theextraction aperture. In certain embodiments, at least one of theplurality of chamber walls is electrically conductive and the bias powersupply is in communication with electrically conductive chamber walls ofthe ion source chamber and the bias power supply provides a negativepulse to the electrically conductive chamber walls. In certainembodiments, the bias power supply is in communication with a platen onwhich the workpiece is disposed, and the bias power supply provides apositive pulse to the platen. In certain embodiments, extraction opticsare disposed outside the ion source chamber and proximate the extractionaperture to manipulate the negative ribbon ion beam. In certainembodiments, a low work function material is disposed on an interiorsurface of at least one of the plurality of chamber walls.

According to another embodiment, a method of extracting a negativeribbon ion beam is disclosed. The method comprises applying a first RFpower level for a first time duration to a RF antenna proximate an ionsource chamber to create a plasma within the ion source chamber from afeed gas and a second RF power level, lower than the first RF powerlevel, for a second time duration; pulsing a bias voltage to attractnegative ions from an ion source chamber, as a negative ribbon ion beam,through an extraction aperture during at least a portion of the secondtime duration; and repeating the applying, and pulsing a plurality oftimes. In certain embodiments, the method further comprises determininga phase delay between an expiration of the first time duration and thepulsing so as to maximize a current of the negative ribbon ion beam. Incertain embodiments, the method further comprises varying the first timeduration, the second time duration, and a phase delay between anexpiration of the first time duration and the pulsing so as to control acomposition of the negative ribbon ion beam.

According to another embodiment, a method of extracting a ribbon ionbeam is disclosed. The method comprises applying a RF power level for afirst time duration from a RF power supply to a RF antenna proximate anion source chamber to create a plasma within the ion source chamber froma feed gas and disabling the RF power supply for a second time durationto reduce an electric field in the ion source chamber; waiting a phasedelay after disabling the RF power supply to allow electrons in the ionsource chamber to cool and to attach to atoms or molecules in the ionsource chamber and create negative ions; and creating a negative voltagedifference between the plasma in the ion source chamber and a workpieceduring a portion of the second time duration so as to attract a negativeribbon ion beam toward the workpiece. In certain embodiments, the phasedelay is selected to maximize a beam current of the negative ribbon ionbeam. In certain embodiments, the negative voltage difference onlyoccurs while the RF power supply is disabled. In certain embodiments,the method further comprises creating a positive voltage differencebetween the plasma in the ion source chamber and a workpiece during aportion of the first time duration, so as to extract a positive ion beamduring the first time duration.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present disclosure, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1A shows an RF ion source according to one embodiment;

FIG. 1B shows an RF ion source according to another embodiment;

FIG. 2A shows a timing diagram used to extract a negative ribbon ionbeam from the RF ion source;

FIG. 2B shows a timing diagram used to extract both a positive ribbonion beam and a negative ribbon ion beam from the RF ion source;

FIG. 3 shows the relationship between bias voltage and negative ion beamcurrent;

FIG. 4A shows the mass spectrum of positive ions extracted using from aRF ion source using hydrogen as the feedgas;

FIG. 4B shows the mass spectrum of negative ions extracted using the RFion source using hydrogen as the feedgas;

FIGS. 5A-5B shows the mass spectrum of positive ions and negative ions,respectively using a second feedgas; and

FIG. 6 illustrates a flowchart showing the creation of a negative ionbeam.

DETAILED DESCRIPTION

As described above, in certain embodiments, negative ion beams arebeneficial. As is well known, traditional ion sources typically generateplasmas that contain mostly positive ions and electrons. To create anegative ion beam, it may be beneficial to extract negative ions duringthe afterglow of the plasma. In the afterglow, the electron density andtemperature decreases to sufficiently low values such that the chargebalance in the plasma is sustained between positive and negative ions.

FIG. 1A shows a first embodiment of an ion source that may be used tocreate a negative ribbon ion beam. In this embodiment, an RF ion source100 is illustrated. The RF ion source 100 comprises a plurality ofchamber walls 111 defining an ion source chamber 110. A first chamberwall 111 a, or a portion thereof, may be constructed of a dielectricmaterial, such as quartz or alumina, forming a dielectric window 112. AnRF antenna 120 may be disposed proximate the first chamber wall 111 a,proximate the dielectric window 112, on the outside of the ion sourcechamber 110. The RF antenna 120 may comprise an electrically conductivematerial, such as copper, which may be wound in a spiral fashion. An RFpower supply 130 is in electrical communication with the RF antenna 120.The RF power supply 130 may supply an RF power to the RF antenna 120.The power supplied by the RF power supply 130 may be between 0.05 and 10kW and may be any suitable frequency, such as between 1 and 40 MHz.Further, the power supplied by the RF power supply 130 may be pulsed, asdescribed in more detail below. For example, the RF power supply 130 maybe capable of generating a plurality of different RF power levels, suchas in a predetermined sequence.

In certain embodiments, certain chamber walls 111, with the exception ofthe dielectric window 112, are electrically conductive, and may beconstructed of a metal or other conductive material. In certainembodiments, these chamber walls 111 may be electrically biased. Incertain embodiments, the chamber walls 111 may be grounded. In otherembodiments, the chamber walls 111 may be biased at a voltage by biaspower supply 140.

In certain embodiments, the bias voltage may be a constant (DC) voltage.In other embodiments, the bias voltage may be pulsed. In theseembodiments, the bias power supply 140 may be a power supply that iscapable of pulsing or otherwise modulating its output. In anotherembodiment, the bias power supply 140 may comprise a traditional powersupply that outputs a fixed voltage. The fixed output of this powersupply may be in communication with a switch or other pulse modulator,which is capable of switching between the output of the power supply andanother voltage, such as ground, for example. Therefore, throughout thisdisclosure, the phrase “bias power supply” is intended to denote asingle power supply that is capable of modulating its output, or acombination of one or more power supplies and a pulse modulator orswitch, that allows different voltages to be created.

The bias voltage applied to the chamber walls 111 establishes thepotential of the plasma within the ion source chamber 110. Thedifference between the electrical potential of the plasma and theelectrical potential of the workpiece 10, referred to as the extractionvoltage, determines which polarity of ions is extracted through theextraction aperture 115. For example, if the plasma is more negativethan the workpiece, negatively charged ions and electrons will beattracted to the workpiece 10.

A second chamber wall 111 b, which may be opposite the first chamberwall 111 a, includes an extraction aperture 115. The extraction aperture115 may be an opening through which the ions generated in the ion sourcechamber 110 are extracted and directed toward a workpiece 10. Theextraction aperture 115 may be any suitable shape. In certainembodiments, the extraction aperture 115 may be oval or rectangularshaped, having one dimension, referred to as the length, which may bemuch larger than the second dimension, referred to as the height. Inthese embodiments, negative ions may be extracted through the extractionaperture 115 as a negative ribbon ion beam 160. The second chamber wall111 b containing the extraction aperture 115 may be made from aconductive material, or an insulating material, such as quartz.

Extraction optics 165 may be disposed between the extraction aperture115 and the workpiece 10. These extraction optics 165 may be used todirect or other manipulate the negative ribbon ion beam 160 as the ionbeam travels toward the workpiece 10. For example, blockers, electrodes,triodes or tetrodes may be used to control the ion beam mean angle andangle spread of the ion beam. In certain embodiments, the extractionoptics may be used to define a specific angle of incidence of thenegative ribbon ion beam 160 relative to the workpiece 10. In someinstances, the angle of incidence may be as high as 7 degrees. Incertain embodiments, an angled negative ribbon ion beam 160 may bebeneficial for patterning or spacer etch.

A gas container 150 may be in communication with the ion source chamber110, such as via a gas inlet 151. The gas container 150 may hold one ormore feed gasses, which are used to create a plasma within the ionsource chamber 110. The feed gasses may comprise any suitable dopant,including but not limited to boron, phosphorus, and arsenic. In certainembodiments, the feed gasses may comprise etching species, such as atomsor molecules containing hydrogen or fluorine. In certain embodiments,the feed gasses may be other species, such as atoms or moleculescontaining Group 3, Group 4 or Group 5 elements. As such, the feed gasthat is introduced into the ion source chamber 110 is not limited bythis disclosure. The flow of gas into the ion source chamber 110 may becontrolled by a mass flow controller disposed proximate the gas inlet151. The pressure within the ion source chamber 110 may be in the rangeof less than 10 mTorr.

Disposed proximate the extraction aperture 115 is the workpiece 10,which may be disposed on a platen 170. The platen 170 may be attached toan actuator to allow the platen 170 to move, for example, in direction171. The platen 170 may be grounded in certain embodiments.

A controller 180, comprising a processing unit 186 and a storage element187, may be in communication with at least the RF power supply 130 andthe bias power supply 140. The controller 180 may actuate the RF powersupply 130 and the bias power supply 140 to create the waveforms shownin FIGS. 2A-2B. The processing unit 186 may be any suitable processor,such as a personal computer, embedded processor, or integrated circuit.The storage element 187 may be any form of non-transitory storage, suchas but not limited to semiconductor memory, magnetic memory, and opticalmemory. The storage element 187 may include instructions that may beexecuted by the processing unit 186. When these instructions areexecuted, the RF ion source 100 may operate as described below.

FIG. 1B shows a second embodiment of the RF ion source 190. FIG. 1B issimilar to the embodiment of FIG. 1A, except that the difference inelectrical potential between the plasma and the workpiece 10 isestablished in a different manner. In this embodiment, like componentshave been given identical reference designators. In FIG. 1B, the chamberwalls 111 may be biased at a fixed voltage, such as ground. The platen170 may be in electrical communication with a bias power supply 140,which is used to bias the platen 170. By varying the voltage applied bythe bias power supply 140 to the platen 170, while maintaining theplasma at ground potential, it is possible to attract positive ions,negative ions, or no charged particles from the ion source chamber 110to the workpiece 10. For example, the application of a positive voltagefrom the bias power supply 140 to the platen 170 will cause negativeions from the ion source chamber 110 to be attracted to the workpiece 10in the form of a negative ribbon ion beam.

Note that to create the same effect, the polarity of the bias voltageapplied to the platen 170 in FIG. 1B is opposite the bias voltageapplied to the chamber walls in FIG. 1A. In other words, a negativevoltage applied to the chamber walls 111 in FIG. 1A will cause negativeions to be attracted to the workpiece 10. However, a positive voltageapplied to the platen 170 in FIG. 1B will cause negative ions to beattracted to the workpiece 10. Thus, throughout this disclosure, theterm “extraction voltage” is defined as the voltage difference betweenthe plasma and the workpiece, expressed as V_(plasma)−V_(workpiece).Thus, a negative extraction voltage denotes a negative bias voltage inFIG. 1A and a positive bias voltage in FIG. 1B.

A controller 180, comprising a processing unit 186 and a storage element187, may be in communication with the RF power supply 130 and the biaspower supply 140. The controller 180 may actuate the RF power supply 130and the bias power supply 140 to create the waveforms shown in FIG.2A-2B. The processing unit 186 and the storage element 187 may be asdescribed above. When the instructions in the storage element 187 areexecuted, the RF ion source 190 operates as described below.

Although FIGS. 1A-1B show a controller 180, it is understood that otherembodiments are also possible. For example, the RF power supply 130 andthe bias power supply 140 may each be in communication with a respectivecontroller. These separate controllers may communicate with each otherto remain synchronized. In other embodiments, these separate controllersmay be synchronized at a single point in time and may operateautonomously thereafter. Thus, throughout this disclosure, the term“controller” refers to the one or more controllers that are used tocontrol the RF power supply 130 and the bias power supply 140. Further,FIGS. 1A-1B show the controller 180 as being a separate component.However, it is understood that the controller 180 may be integrated intoanother component. For example, the controller 180 may be integratedinto the RF power supply 130 or the bias power supply 140. In otherwords, the figures are intended to represent the various functionsperformed by the RF ion source 100, however, these functions may bearranged in various configurations. The configuration shown in FIG.1A-1B is but one possible arrangement of the components.

In operation, feed gas from gas container 150 is introduced to the ionsource chamber 110 via gas inlet 151. An RF power is applied by the RFpower supply 130 to the RF antenna 120. This RF power energizes the feedgas within the ion source chamber 110, producing a plasma, whichcontains mostly positive ions and electrons. By creating a voltagedifferential between the plasma and the workpiece 10 (i.e. a non-zeroextraction voltage), ions are extracted from the ion source chamber 110through the extraction aperture 115 as a ribbon ion beam and impact theworkpiece 10. As described above, this extraction voltage may be createdby varying the voltage applied by bias power supply 140 to the chamberwalls 111, as shown in FIG. 1A. Alternatively, the extraction voltagemay be created by applying a bias voltage to the platen 170 using biaspower supply 140, as shown in FIG. 1B.

As described above, the RF power supply 130 may operate in a pulsed ormodulated mode, such that the RF power supply 130 applies a first RFpower level to the RF antenna 120 during a first time duration, and thenapplies a second RF power level to the RF antenna 120 during a secondtime duration. The second RF power level may be less than the first RFpower level. In certain embodiments, the second RF power level is 0volts. In certain embodiments where the second RF power level is greaterthan 0 volts, a mechanism to filter extracted electrons may be used. Thesum of the first time duration and the second time duration defines theperiod of the RF power supply 130. In some embodiments, the period ofthe RF power supply may be 200 μsec, although other time durations mayalso be used. The duty cycle of the RF power supply 130 is defined asthe ratio of the first time duration to the period of the RF powersupply 130. In some embodiments, the first time duration and the secondtime duration may be of equal duration, such that the RF power supply130 has a 50% duty cycle. However, other duty cycles are also within thescope of the disclosure. The power applied by the RF power supply 130during the first time duration may be between 50 W and 10 kW, althoughother power levels may also be used. The first time duration may bebetween 25 μsec and 10 milliseconds.

FIG. 2A shows a timing diagram, wherein line 200 shows a representativepower waveform output by the RF power supply 130. In this embodiment,the duty cycle of the RF power supply 130 is 50%, although other valuesmay be used. The period of this waveform may be between 100 μsec and 10milliseconds. Line 210 is representative of the number of electrons inthe plasma. Line 220 is representative of the number of negative ions inthe plasma. Note that while the RF power supply 130 outputs the first RFpower level, the number of electrons is greater than the number ofnegative ions, as the energy of the RF antenna causes the creation of aplasma that comprises mostly electrons and positive ions.

When the RF power supply 130 outputs the second RF power level, whichmay be 0 volts in this example, the plasma cools, and the number ofelectrons, as shown in line 210, decreases in the plasma afterglow.Meanwhile, the number of negative ions actually increases when the RFpower supply 130 outputs the second RF power level, forming an ion-ionplasma. Line 230 represents the voltage difference between the voltageof the plasma and the voltage of the workpiece 10, referred to as theextraction voltage and expressed as V_(plasma)−V_(workpiece). Thisextraction voltage is created using controller 180, by the synchronizedpulsing of the output of bias power supply 140. The period of the biaspower supply waveform may be the same as the period of the RF powersupply 130. When the extraction voltage is zero, ions are not attractedtoward the workpiece 10. When this extraction voltage is negative, theplasma is more negative than the workpiece 10, and negative ions andelectrons are attracted to the workpiece 10. By creating a negativeextraction voltage between the plasma and the workpiece when the RFpower supply 130 is not actuated, it is possible to create a negativeribbon ion beam 160. Further, by introducing a phase delay between thetime when the RF power supply 130 transitions to the second RF powerlevel and the creation of this voltage difference, the ratio of negativeions to electrons may be maximized. The controller 180 may coordinatethe actions of the RF power supply 130 and the bias power supply 140 toachieve these waveforms. As described above, in certain embodiments, twoseparate controllers are used to control the RF power supply 130 and thebias power supply 140.

The duty cycle of the bias power supply 140 is defined as the ratio ofthe pulse width of the extraction voltage to the period of the biaspower supply 140, which may equal the period of the RF power supply 130.In certain embodiments, the duty cycle of the RF power supply 130 may be50%, while the duty cycle of the bias power supply 140 may be about 20%.In certain embodiments, the duty cycle of the bias power supply 140 maybe based on the duty cycle of the RF power cycle. In certainembodiments, the duty cycle of the RF power supply 130 may be such thatthe entire pulse of the bias power supply 140 occurs during the secondtime duration while the RF power supply 130 outputs the second RF powerlevel.

Further, assume the RF power supply 130 outputs the first RF power levelat the beginning of the period, and outputs the second RF power level ata point that is 50% through the period. In this case, the controller 180may pulse the bias power supply 140 at a point at least 65% through theperiod. In other words, the bias power supply 140 may be pulsed afterdelaying from the output of the second RF power level from the RF powersupply 130. The pulse for the bias power supply 140 may be deactivatedprior to the next point in time when the RF power supply 130 outputs thefirst RF power level. In other words, in certain embodiments, the entirepulse width of the bias power supply 140 occurs while the RF powersupply 130 is outputting the second RF power level.

By varying the synchronous pulsed plasma parameters, such as the periodof the RF power supply 130, the duty cycle of the RF power supply 130,the duty cycle of the bias power supply 140 (i.e. the pulse width) andphase delay between the output of the second RF power level and thesynchronized pulsing of the bias power supply 140, the negative ioncurrent intensity may be optimized and the transition from electron-ionto ion-ion plasma may be controlled.

The waveforms shown in FIG. 2A may be used to extract a negative ribbonion beam to direct toward a workpiece 10. This negative ribbon ion beammay be used for etching processes, deposition processes or implantprocesses.

FIG. 2B shows a second representative timing diagram. In thisembodiment, both a positive ribbon ion beam and a negative ribbon ionbeam are extracted from the ion source chamber 110. As described above,line 200 represents the RF power applied to the RF antenna 120 by the RFpower supply 130. Lines 210, 220 represent the relative number ofelectrons and negative ions, respectively. Line 240 represents thevoltage difference between the plasma and the workpiece 10, as known asthe extraction voltage and is expressed as V_(plasma)−V_(workpiece). Inthis embodiment, the controller 180 actuates the bias power supply 140twice during each period. The first pulse is a positive extractionvoltage, which causes positive ions from the ion source chamber 110 tobe attracted to the workpiece 10. This first pulse is created when theRF power supply 130 is actuated at the first RF power level. This firstpulse may be actuated anytime during the time that the RF power supply130 is actuated at the first RF power level. The second pulse is anegative extraction voltage, which causes negative ions from the ionsource chamber 110 to be attracted to the workpiece 10. As describedabove with respect to FIG. 2A, the second pulse is created when the RFpower supply 130 is actuated at the second RF power level.

In certain embodiments, the positive ribbon ion beams and negativeribbon ion beams can both be used to perform deposition, etching orimplantation processes. Additionally, the use of both ribbon ion beamsmay reduce the accumulation of charge on the workpiece 10.

The positive and negative ion beams may have the same energy, or inother embodiments, may have different energies. In certain embodiments,both ion beams are delivered to the workpiece for processing. In otherembodiments, the ion beams are delivered at different times during theprocessing of the workpiece.

This apparatus and method allows the generation of negative ribbon ionbeams. FIG. 3 shows a timing diagram showing the relationship betweenbias voltage and negative ion beam current. Line 300 represents theextraction voltage applied by the bias power supply 140. Line 310 showsthe current of the extracted negative ribbon ion beam. In this figure,the period of the bias power supply is about 200 μsec, although otherdurations may be used. This graph represents ion beam current that maybe generated using hydrogen as the feed gas. The other parameters, suchas RF power level, RF duty cycle, and phase delay are in the rangedescribed above. The extraction voltage, or the voltage differencebetween the voltage of the plasma and the voltage of the workpiece 10,may be about 1 kV. This graph shows that a negative hydrogen ion beam isextracted during the pulses supplied by the bias power supply 140. Themagnitude of the extracted ion beam may be roughly 1.4 mA/cm².

In some embodiments, the ion beam current of the negative ribbon ionbeam may be improved. For example, it may be possible to coat thechamber walls 111 with a low work function material, such as tantalum.In certain embodiments, the regions around the extraction aperture 115may be coated with the low work function material. The bombardment ofthis low work function material increases the surface temperature of thechamber walls 111 to roughly 300° C. or more, and may enhance surfaceionization and secondary negative ion formation. In other words, thematerial used to coat the chamber walls 111 may contribute electrons tothe plasma.

Additionally, this negative ribbon ion beam may be manipulated in muchthe same way that positive ion beams are manipulated. Specifically, asdescribed above, extraction optics 165 may be disposed outside the ionsource chamber 110 proximate the extraction aperture 115. Theseextraction optics 165 may be used to allow the negative ribbon ion beamto strike the workpiece 10 at a specific incident angle.

Unexpectedly, the mass spectrum of negative ions may be different fromthe mass spectrum of positive ions for the same feed gas at similar gaspressure and RF power levels. Electron attachment processes in theplasma and on the chamber walls control negative ion production, whichlimits the ion flux from weakly electronegative plasmas. This fact maybe used to control the species created and extracted from the ion sourcechamber 110.

For example, FIG. 4A shows the mass spectrum of positive ions extractedin one configuration. In this case, the feed gas was hydrogen, theextraction voltage was 1 kV, and the RF power supply 130 was operatedcontinuously. FIG. 4A shows that positive ions of H⁺, H₂ ⁺ and H₃ ⁺ areall generated in this configuration. Further, H₃ ⁺ ions were the mostcommon, having almost three times more current than the H⁺ and H₂ ⁺ions. Of course, the most abundant ion may change as a result ofdifferences in gas pressure and RF power levels.

In FIG. 4B, a negative ion beam was created using hydrogen as a feedgas. In this test, the output of the RF power supply 130 was modulatedat a 50% duty cycle, where the second RF power level was 0 volts. Thebias power supply 140 was operated with a duty cycle of 20%. In thistest, the duty cycle of the RF power supply 130 was 200 μsec, and thebias power supply was pulsed after the RF power supply 130 wasdeactivated. In this test, almost all of the negative ions created wereH⁻ ions. In fact, the current of H₂ ⁻ and H₃ ⁻ ions was too small to bemeasured. In other words, the species extracted as a negative ion beamis limited to only H⁻ ions, while the positive ion beam has threedifferent species with different masses. Mass spectra were determinedusing a magnetic sector with a polarity switch that enables detection ofpositive and negative ion species in conjunction with a Faraday cup.

However, the positive ion beam current, shown in FIG. 4A, was about 10times greater than the negative ion beam current shown in FIG. 4B.

FIGS. 5A-5B show a second configuration, where CH₃F and O₂ were used asthe feed gas. In the case of positive ions, as shown in FIG. 5A, a largenumber of different species, include H⁺, H₂ ⁺, C⁺, F⁺, HF⁺, H₂F⁺, H₃F⁺,CF⁺ and CH₃F⁺, were generated. In contrast, a much smaller number ofnegative species were generated. FIG. 5B shows the currents for H⁻, C⁻,CH⁻ and F⁻ ions. All other species were generated in an amount too smallto be measured.

The smaller number of species that are created as negative ions maysimplify the design of the ion source. For example, in certainembodiments, it may be possible to eliminate the mass analyzer, so thatall extracted ions are implanted in the workpiece. Thus, by manipulatingthe duty cycle and period of the RF power supply 130, the duty cycle ofthe bias power supply 140 and the phase delay, the composition of theextracted negative ribbon ion beam can be controlled. This may allow areduction of elimination of mass analysis in certain embodiments.However, in other embodiments, a mass analyzer may be used inconjunction with the RF ion source described herein.

FIG. 6 shows a flowchart that may be executed by the controller 180 togenerate a negative ion beam. In certain embodiments, a non-transitorymedia may contain a software program, which when loaded into the storageelement 187 of the controller 180, causes the controller 180 to executethese processes.

First, as shown in Process 600, the controller 180 actuates the RF powersupply 130 so that the RF power supply outputs a first RF power having afrequency of between 1 and 20 MHz, and has an output power of, forexample, between 0.050 kW and 5 kW. The RF power supply 130 may outputthe first RF power level for a first time duration. This first timeduration may be between 100 μsec and 10 milliseconds, such as 100 μsec.The first RF power level causes the feed gas in the ion source chamber110 to form a plasma, which is mainly positive ions and electrons.

After the expiration of the first time duration, the controller 180causes the RF power supply 130 to output a second RF power level, asshown in Process 610. This deenergizes the plasma in the ion sourcechamber 110 and cools the electrons, which leads to a reduction in thenumber of electrons in the ion source chamber 110 and an increase in thenumber of negative ions. In certain embodiments, the second RF powerlevel is 0 volts. As described above, in embodiments where the second RFpower level is greater than 0 volts, electrons may be co-extracted withthe negative ions. In these embodiments, electron filtering may beperformed outside the extraction aperture 115 to deflect or separate theelectrons from the negative ions to obtain a pure negative ribbon ionbeam.

The controller 180 then waits a phase delay, as shown in Process 620.This phase delay may be as small as 0 μsec, or may be much larger, suchas several milliseconds. The purpose of the phase delay is to allow theelectrons in the plasma to dissipate, and the number of negative ions toincrease. The phase delay is a parameter that may be tuned forcontrolling the attachment process in the plasma. Thus, negative ioncurrent can be optimized using the phase delay. For example, maximumextracted negative ion current (during the bias pulse) may be a strongfunction of the initial electron temperature and plasma density.Electron attachment processes in the afterglow are maximized for acertain initial electron temperature and plasma density. For example, ata particular set of parameters, the phase delay can be used to shape thecurrent pulse and intensity of the negative ion current. Theseparameters may include the period of the RF power supply, the duty cycleor first time duration of the RF power supply, the duty cycle of thebias power supply, the species of the feed gas, the gas process withinthe ion source chamber. In certain embodiments, the first RF power leveland the phase delay define the initial and final electron temperatures.

After the phase delay, the controller 180 may pulse the bias powersupply to create a negative extraction voltage for a second timeduration, as shown in Process 630. While there is a negative extractionvoltage, negative ions are attracted from the ion source chamber 110 tothe workpiece 10, which is less negatively biased. Thus, it is duringthis pulse that the negative ribbon ion beam is extracted through theextraction aperture 115. This second time duration may be any suitabletime, such as 40 μsec, although other values are also possible.

After pulsing the extraction voltage, the controller 180 waits a delaytime, as shown in Process 640. After the delay time, the sequencerepeats by returning to Process 600.

The sum of the first time duration, the second time duration, the phasedelay and the delay time defines the period of the RF power supply 130.The ratio of the first time duration to the period of the RF powersupply 130 defines the duty cycle of the RF power supply. In someembodiments, the first time duration may be equal to the sum of thesecond time duration, the phase delay and the delay time, such that theRF power supply 130 has a 50% duty cycle. However, in other embodiments,the RF power supply may have a different duty cycle, such between 5 and95%.

The bias power supply 140 operates using the same period as the RF powersupply 130. The ratio of the second time duration to the period definesthe duty cycle of the bias power supply 140. In certain embodiments, theextraction voltage is pulsed for about 20% of the period of the RF powersupply 130. However, other values are also possible.

In certain embodiments, the phase delay is determined based on the RFpower level, the gas pressure within the ion source chamber 110, theperiod of the RF power supply and the feed gas used. In certainembodiments, the phase delay may be determined empirically to maximizethe beam current.

In summary, the controller 180 operates the RF power supply 130 and thebias power supply 140 such that each has the same period. However, theduty cycle and phase of each is different from the other to achieve theresulting negative ion beam. For example, the RF power supply 130 mayhave a period and a first duty cycle. The bias power supply 140 may havethe same period and a second duty cycle. Further, the phase differencebetween these two power supplies is such that the bias power supply 140is pulsed while the RF power supply outputs the second RF power level(i.e. may be in the “off” state).

Although not shown in FIG. 6, in certain embodiments, the controller 180may pulse a positive extraction voltage during a portion of Process 600to extract a positive ion beam from the ion source chamber 110.

As described above, in certain embodiments, the RF power supply 130 andthe bias power supply 140 may be regulated using two separatecontrollers. In this embodiment, the RF controller is responsible forthe period and duty cycle of the RF power supply 130, while the biascontroller is responsible for the period and duty of the bias powersupply 140. These two controllers may be synchronized to establish thephase delay between their outputs.

For example, the negative ribbon ion beam may be extracted by using theRF power supply 130 to repeatedly apply a first RF power level to the RFantenna for a first time duration and a second RF power level to the RFantenna for a second time duration. The bias power supply 140 may besynchronously pulsed to attract negative ions from the ion sourcechamber, in the form of a negative ribbon ion beam, through theextraction aperture during at least a portion of the second timeduration. Further, in certain embodiments, the phase delay between theexpiration of the first time duration and the pulsing is determined soas to maximize the current of the negative ribbon ion beam. In certainembodiments, the first time duration, the second time duration and thephase delay are varied so as to control the composition of the negativeribbon ion beams, in terms of the extracted negative species.

The present apparatus has many advantages. First, in certainapplications, negative ion beams are preferred. For example, negativeion beams may be beneficial for etching and deposition processes.Secondly, the mass spectrum of negative ion beams may differ from thatof a similarly generated positive ion beam. This may allow theelimination of mass analysis in certain embodiments. Additionally, bothpositive and negative ion beams can be extracted from a single ionsource, which may be beneficial in etching of next generationmicro-devices having a characteristic feature size that is less than 10nm. A negative ribbon ion beam at a small incident angle may beespecially beneficial for the Si₃N₄ spacer etch and gate patterning.Negative ion beams could also enable other processes, such ascharge-free plasma processing, high etch selectivity and anisotropy,etch uniformity and others.

In addition, the selection of the various parameters, such as the periodand duty cycle of the RF power supply 130, the duty cycle of the biaspower supply 140, and the phase delay, enables control of the plasmapotential, the electron temperature, the ion to electron flux ratio andthe ion to neutral flux ratio.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Furthermore, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

What is claimed is:
 1. A method of extracting a negative ribbon ion beamfrom an ion source chamber at a particular angle, comprising: applying afirst RF power level for a first time duration to a RF antenna proximatean ion source chamber to create a plasma within the ion source chamberfrom a feed gas and a second RF power level, lower than the first RFpower level, for a second time duration, wherein the plasma cools duringthe second time duration to create an afterglow plasma in which a numberof negative ions in the plasma increases; pulsing a bias voltage toattract negative ions from an ion source chamber, as a negative ribbonion beam, through an extraction aperture during at least a portion ofthe second time duration; and using extraction optics disposed outsideof the ion source chamber between the extraction aperture and aworkpiece to define a specific angle of incidence of the negative ribbonion beam relative to a workpiece.
 2. The method of claim 1, wherein theextraction optics comprises blockers, electrodes, triodes or tetrodes.3. The method of claim 1, wherein the extraction optics also control anion beam mean angle and angle spread of the negative ribbon ion beam. 4.The method of claim 1, wherein the specific angle of incidence is ashigh as 7 degrees.
 5. The method of claim 1, wherein the second RF powerlevel is ground.
 6. An apparatus for creating a negative ribbon ion beamand a positive ribbon ion beam, comprising: an ion source having aplurality of chamber walls defining an ion source chamber and having anextraction aperture; an RF antenna disposed proximate one of theplurality of chamber walls of the ion source chamber; an RF power supplyin communication with the RF antenna, and outputting a first RF powerlevel for a first time duration to the RF antenna to create a plasmawithin the ion source chamber from a feed gas and outputting a second RFpower level, lower than the first RF power level, for a second timeduration, wherein the second RF power level is selected such that theplasma cools during the second time duration to create an afterglowplasma in which a number of negative ions in the plasma increases; and abias power supply to create a positive extraction voltage during atleast a portion of the first time duration to extract the positiveribbon ion beam from the ion source chamber and a negative extractionvoltage during at least a portion of the second time duration to extractthe negative ribbon ion beam from the ion source chamber, whereinextraction voltage is defined as a voltage difference between the plasmadisposed in the ion source chamber and a workpiece.
 7. The apparatus ofclaim 6, wherein at least one of the plurality of chamber walls iselectrically conductive and the bias power supply is in communicationwith electrically conductive chamber walls of the ion source chamber andthe bias power supply provides pulses to the electrically conductivechamber walls.
 8. The apparatus of claim 6, wherein the bias powersupply is in communication with a platen on which the workpiece isdisposed, and the bias power supply provides pulses to the platen. 9.The apparatus of claim 6, comprising extraction optics disposed outsidethe ion source chamber and proximate the extraction aperture tomanipulate the positive ribbon ion beam and the negative ribbon ionbeam.
 10. The apparatus of claim 6, further comprising a coatingcomprising a low work function material, disposed on an interior surfaceof at least one of the plurality of chamber walls to contributeelectrons to the plasma.
 11. The apparatus of claim 6, wherein thesecond RF power level is 0 volts.
 12. The apparatus of claim 6, whereinthe bias power supply delays a phase delay after an expiration of thefirst time duration before creating the negative extraction voltage. 13.The apparatus of claim 12, wherein the phase delay is selected so as tomaximize a beam current of the negative ribbon ion beam.
 14. Theapparatus of claim 6, wherein the first time duration and the secondtime duration define a period of the RF power supply, and the RF powersupply repeatedly outputs the first RF power level and the second RFpower level, and wherein the bias power supply continuously outputs thepositive extraction voltage and negative extraction voltage, and has aperiod equal to the period of the RF power supply.